1. Field of the Invention
The present invention relates to a device for providing a camera system with an information necessary for automatically controlling the focus, or indicating the condition of the focus adjustment to aid the manual focus control.
2. Description of the Prior Art
In this field of art to provide an information for a camera focus adjustment, there has been known a device utilizing Charge Coupled Device (CCD) which has a plurality of photosensitive elements each capable of accumulating electric charges generated in response to light fall on each of the photosensitive elements, and means for taking out the electric charges accumulated in each of the photosensitive elements to serially produce electric signals corresponding thereto. Since the accumulation of electric charges makes less progress for less light falling on the photosensitive elements, the device is provided with means for causing the taking out of the electric charges by the taking out means at a lapse of a time variable in response to the light to be measured to prolong the time with a greater degree for the lower measured light level. According to this control of accumulating time, the average of amounts of the taken out electric charges is always the same, which is favorable to process the electric signals produced by the taking out means when the average intensity of the measured scene widely varies.
According to the above device for providing an information for a focus adjustment, however, it would take a considerable time for the device to complete the desired level of electric charge accumulation if the average intensity of the measured scene is remarkably low. This disadvantage in low light intensity would cause a slow response of focus adjustment.